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<body class="manpage">
<div id="header">
<h1>
zmq_socket(3) Manual Page
</h1>
<h2>NAME</h2>
<div class="sectionbody">
<p>zmq_socket -
   create 0MQ socket
</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="paragraph"><p><strong>void *zmq_socket (void <em>*context</em>, int <em>type</em>);</strong></p></div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <em>zmq_socket()</em> function shall create a ØMQ socket within the specified
<em>context</em> and return an opaque handle to the newly created socket. The <em>type</em>
argument specifies the socket type, which determines the semantics of
communication over the socket.</p></div>
<div class="paragraph"><p>The newly created socket is initially unbound, and not associated with any
endpoints. In order to establish a message flow a socket must first be
connected to at least one endpoint with <a href="zmq_connect.html">zmq_connect(3)</a>, or at least one
endpoint must be created for accepting incoming connections with
<a href="zmq_bind.html">zmq_bind(3)</a>.</p></div>
<div class="paragraph"><div class="title">Key differences to conventional sockets</div><p>Generally speaking, conventional sockets present a <em>synchronous</em> interface to
either connection-oriented reliable byte streams (SOCK_STREAM), or
connection-less unreliable datagrams (SOCK_DGRAM). In comparison, ØMQ sockets
present an abstraction of an asynchronous <em>message queue</em>, with the exact
queueing semantics depending on the socket type in use. Where conventional
sockets transfer streams of bytes or discrete datagrams, ØMQ sockets transfer
discrete <em>messages</em>.</p></div>
<div class="paragraph"><p>ØMQ sockets being <em>asynchronous</em> means that the timings of the physical
connection setup and tear down, reconnect and effective delivery are transparent
to the user and organized by ØMQ itself. Further, messages may be <em>queued</em> in
the event that a peer is unavailable to receive them.</p></div>
<div class="paragraph"><p>Conventional sockets allow only strict one-to-one (two peers), many-to-one
(many clients, one server), or in some cases one-to-many (multicast)
relationships. With the exception of <em>ZMQ_PAIR</em>, ØMQ sockets may be connected
<strong>to multiple endpoints</strong> using <em>zmq_connect()</em>, while simultaneously accepting
incoming connections <strong>from multiple endpoints</strong> bound to the socket using
<em>zmq_bind()</em>, thus allowing many-to-many relationships.</p></div>
<div class="paragraph"><div class="title">Thread safety</div><p>ØMQ <em>sockets</em> are <em>not</em> thread safe. Applications MUST NOT use a socket
from multiple threads except after migrating a socket from one thread to
another with a "full fence" memory barrier.</p></div>
<div class="paragraph"><div class="title">Socket types</div><p>The following sections present the socket types defined by ØMQ, grouped by the
general <em>messaging pattern</em> which is built from related socket types.</p></div>
<div class="sect2">
<h3 id="_request_reply_pattern">Request-reply pattern</h3>
<div class="paragraph"><p>The request-reply pattern is used for sending requests from a ZMQ_REQ <em>client</em>
to one or more ZMQ_REP <em>services</em>, and receiving subsequent replies to each
request sent.</p></div>
<div class="sect3">
<h4 id="_zmq_req">ZMQ_REQ</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_REQ</em> is used by a <em>client</em> to send requests to and
receive replies from a <em>service</em>. This socket type allows only an alternating
sequence of <em>zmq_send(request)</em> and subsequent <em>zmq_recv(reply)</em> calls. Each
request sent is round-robined among all <em>services</em>, and each reply received is
matched with the last issued request.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_REQ</em> socket enters the <em>mute</em> state due to having reached the
high water mark for all <em>services</em>, or if there are no <em>services</em> at all, then
any <a href="zmq_send.html">zmq_send(3)</a> operations on the socket shall block until the
<em>mute</em> state ends or at least one <em>service</em> becomes available for sending;
messages are not discarded.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_REQ characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_REP</em>, <em>ZMQ_ROUTER</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Bidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Send, Receive, Send, Receive, &#8230;
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Round-robin
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Last peer
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Block
</p>
</td>
</tr>
</table></div>
</div>
<div class="sect3">
<h4 id="_zmq_rep">ZMQ_REP</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_REP</em> is used by a <em>service</em> to receive requests from and
send replies to a <em>client</em>. This socket type allows only an alternating
sequence of <em>zmq_recv(request)</em> and subsequent <em>zmq_send(reply)</em> calls. Each
request received is fair-queued from among all <em>clients</em>, and each reply sent
is routed to the <em>client</em> that issued the last request. If the original
requester doesn&#8217;t exist any more the reply is silently discarded.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_REP</em> socket enters the <em>mute</em> state due to having reached the
high water mark for a <em>client</em>, then any replies sent to the <em>client</em> in
question shall be dropped until the mute state ends.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_REP characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_REQ</em>, <em>ZMQ_DEALER</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Bidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Receive, Send, Receive, Send, &#8230;
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Last peer
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Drop
</p>
</td>
</tr>
</table></div>
</div>
<div class="sect3">
<h4 id="_zmq_dealer">ZMQ_DEALER</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_DEALER</em> is an advanced pattern used for extending
request/reply sockets. Each message sent is round-robined among all connected
peers, and each message received is fair-queued from all connected peers.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_DEALER</em> socket enters the <em>mute</em> state due to having reached the
high water mark for all peers, or if there are no peers at all, then any
<a href="zmq_send.html">zmq_send(3)</a> operations on the socket shall block until the mute
state ends or at least one peer becomes available for sending; messages are not
discarded.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_DEALER</em> socket is connected to a <em>ZMQ_REP</em> socket each message sent
must consist of an empty message part, the <em>delimiter</em>, followed by one or more
<em>body parts</em>.</p></div>
<div class="paragraph"><p>Deprecated alias: <em>ZMQ_XREQ</em>.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_DEALER characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_ROUTER</em>, <em>ZMQ_REP</em>, <em>ZMQ_DEALER</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Bidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unrestricted
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Round-robin
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Block
</p>
</td>
</tr>
</table></div>
</div>
<div class="sect3">
<h4 id="_zmq_router">ZMQ_ROUTER</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_ROUTER</em> is an advanced socket type used for extending
request/reply sockets. When receiving messages a <em>ZMQ_ROUTER</em> socket shall
prepend a message part containing the <em>identity</em> of the originating peer to the
message before passing it to the application. Messages received are fair-queued
from among all connected peers. When sending messages a <em>ZMQ_ROUTER</em> socket shall
remove the first part of the message and use it to determine the <em>identity</em> of
the peer the message shall be routed to. If the peer does not exist anymore
the message shall be silently discarded by default, unless <em>ZMQ_ROUTER_BEHAVIOR</em>
socket option is set to <em>1</em>.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_ROUTER</em> socket enters the <em>mute</em> state due to having reached the
high water mark for all peers, then any messages sent to the socket shall be dropped
until the mute state ends. Likewise, any messages routed to a peer for which
the individual high water mark has been reached shall also be dropped.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_REQ</em> socket is connected to a <em>ZMQ_ROUTER</em> socket, in addition to the
<em>identity</em> of the originating peer each message received shall contain an empty
<em>delimiter</em> message part. Hence, the entire structure of each received message
as seen by the application becomes: one or more <em>identity</em> parts, <em>delimiter</em>
part, one or more <em>body parts</em>. When sending replies to a <em>ZMQ_REQ</em> socket the
application must include the <em>delimiter</em> part.</p></div>
<div class="paragraph"><p>Deprecated alias: <em>ZMQ_XREP</em>.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_ROUTER characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_DEALER</em>, <em>ZMQ_REQ</em>, <em>ZMQ_ROUTER</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Bidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unrestricted
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
See text
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Drop
</p>
</td>
</tr>
</table></div>
</div>
</div>
<div class="sect2">
<h3 id="_publish_subscribe_pattern">Publish-subscribe pattern</h3>
<div class="paragraph"><p>The publish-subscribe pattern is used for one-to-many distribution of data from
a single <em>publisher</em> to multiple <em>subscribers</em> in a fan out fashion.</p></div>
<div class="sect3">
<h4 id="_zmq_pub">ZMQ_PUB</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_PUB</em> is used by a <em>publisher</em> to distribute data.
Messages sent are distributed in a fan out fashion to all connected peers.
The <a href="zmq_recv.html">zmq_recv(3)</a> function is not implemented for this socket type.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_PUB</em> socket enters the <em>mute</em> state due to having reached the
high water mark for a <em>subscriber</em>, then any messages that would be sent to the
<em>subscriber</em> in question shall instead be dropped until the mute state
ends. The <em>zmq_send()</em> function shall never block for this socket type.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_PUB characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_SUB</em>, <em>ZMQ_XSUB</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Send only
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fan out
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Drop
</p>
</td>
</tr>
</table></div>
</div>
<div class="sect3">
<h4 id="_zmq_sub">ZMQ_SUB</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_SUB</em> is used by a <em>subscriber</em> to subscribe to data
distributed by a <em>publisher</em>. Initially a <em>ZMQ_SUB</em> socket is not subscribed to
any messages, use the <em>ZMQ_SUBSCRIBE</em> option of <a href="zmq_setsockopt.html">zmq_setsockopt(3)</a> to
specify which messages to subscribe to. The <em>zmq_send()</em> function is not
implemented for this socket type.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_SUB characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_PUB</em>, <em>ZMQ_XPUB</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Receive only
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Drop
</p>
</td>
</tr>
</table></div>
</div>
<div class="sect3">
<h4 id="_zmq_xpub">ZMQ_XPUB</h4>
<div class="paragraph"><p>Same as ZMQ_PUB except that you can receive subscriptions from the peers
in form of incoming messages. Subscription message is a byte 1 (for
subscriptions) or byte 0 (for unsubscriptions) followed by the subscription
body.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_XPUB characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_SUB</em>, <em>ZMQ_XSUB</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Send messages, receive subscriptions
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fan out
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Drop
</p>
</td>
</tr>
</table></div>
</div>
<div class="sect3">
<h4 id="_zmq_xsub">ZMQ_XSUB</h4>
<div class="paragraph"><p>Same as ZMQ_SUB except that you subscribe by sending subscription messages to
the socket. Subscription message is a byte 1 (for subscriptions) or byte 0
(for unsubscriptions) followed by the subscription body.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_XSUB characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_PUB</em>, <em>ZMQ_XPUB</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Receive messages, send subscriptions
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Drop
</p>
</td>
</tr>
</table></div>
</div>
</div>
<div class="sect2">
<h3 id="_pipeline_pattern">Pipeline pattern</h3>
<div class="paragraph"><p>The pipeline pattern is used for distributing data to <em>nodes</em> arranged in
a pipeline. Data always flows down the pipeline, and each stage of the pipeline
is connected to at least one <em>node</em>. When a pipeline stage is connected to
multiple <em>nodes</em> data is round-robined among all connected <em>nodes</em>.</p></div>
<div class="sect3">
<h4 id="_zmq_push">ZMQ_PUSH</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_PUSH</em> is used by a pipeline <em>node</em> to send messages
to downstream pipeline <em>nodes</em>. Messages are round-robined to all connected
downstream <em>nodes</em>. The <em>zmq_recv()</em> function is not implemented for this
socket type.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_PUSH</em> socket enters the <em>mute</em> state due to having reached the
high water mark for all downstream <em>nodes</em>, or if there are no downstream
<em>nodes</em> at all, then any <a href="zmq_send.html">zmq_send(3)</a> operations on the socket shall
block until the mute state ends or at least one downstream <em>node</em>
becomes available for sending; messages are not discarded.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_PUSH characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_PULL</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Send only
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Round-robin
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Block
</p>
</td>
</tr>
</table></div>
</div>
<div class="sect3">
<h4 id="_zmq_pull">ZMQ_PULL</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_PULL</em> is used by a pipeline <em>node</em> to receive messages
from upstream pipeline <em>nodes</em>. Messages are fair-queued from among all
connected upstream <em>nodes</em>. The <em>zmq_send()</em> function is not implemented for
this socket type.</p></div>
<div class="hdlist"><div class="title">Summary of ZMQ_PULL characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_PUSH</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Receive only
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Fair-queued
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Block
</p>
</td>
</tr>
</table></div>
</div>
</div>
<div class="sect2">
<h3 id="_exclusive_pair_pattern">Exclusive pair pattern</h3>
<div class="paragraph"><p>The exclusive pair pattern is used to connect a peer to precisely one other
peer. This pattern is used for inter-thread communication across the inproc
transport.</p></div>
<div class="sect3">
<h4 id="_zmq_pair">ZMQ_PAIR</h4>
<div class="paragraph"><p>A socket of type <em>ZMQ_PAIR</em> can only be connected to a single peer at any one
time.  No message routing or filtering is performed on messages sent over a
<em>ZMQ_PAIR</em> socket.</p></div>
<div class="paragraph"><p>When a <em>ZMQ_PAIR</em> socket enters the <em>mute</em> state due to having reached the
high water mark for the connected peer, or if no peer is connected, then
any <a href="zmq_send.html">zmq_send(3)</a> operations on the socket shall block until the peer
becomes available for sending; messages are not discarded.</p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content"><em>ZMQ_PAIR</em> sockets are designed for inter-thread communication across
the <a href="zmq_inproc.html">zmq_inproc(7)</a> transport and do not implement functionality such
as auto-reconnection. <em>ZMQ_PAIR</em> sockets are considered experimental and may
have other missing or broken aspects.</td>
</tr></table>
</div>
<div class="hdlist"><div class="title">Summary of ZMQ_PAIR characteristics</div><table>
<tr>
<td class="hdlist1">
Compatible peer sockets
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
<em>ZMQ_PAIR</em>
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Direction
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Bidirectional
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Send/receive pattern
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Unrestricted
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Incoming routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Outgoing routing strategy
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
N/A
</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Action in mute state
<br />
</td>
<td class="hdlist2">
<p style="margin-top: 0;">
Block
</p>
</td>
</tr>
</table></div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_return_value">RETURN VALUE</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <em>zmq_socket()</em> function shall return an opaque handle to the newly created
socket if successful. Otherwise, it shall return NULL and set <em>errno</em> to one of
the values defined below.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_errors">ERRORS</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
<strong>EINVAL</strong>
</dt>
<dd>
<p>
The requested socket <em>type</em> is invalid.
</p>
</dd>
<dt class="hdlist1">
<strong>EFAULT</strong>
</dt>
<dd>
<p>
The provided <em>context</em> is invalid.
</p>
</dd>
<dt class="hdlist1">
<strong>EMFILE</strong>
</dt>
<dd>
<p>
The limit on the total number of open ØMQ sockets has been reached.
</p>
</dd>
<dt class="hdlist1">
<strong>ETERM</strong>
</dt>
<dd>
<p>
The context specified was terminated.
</p>
</dd>
</dl></div>
</div>
</div>
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<h2 id="_see_also">SEE ALSO</h2>
<div class="sectionbody">
<div class="paragraph"><p><a href="zmq_init.html">zmq_init(3)</a>
<a href="zmq_setsockopt.html">zmq_setsockopt(3)</a>
<a href="zmq_bind.html">zmq_bind(3)</a>
<a href="zmq_connect.html">zmq_connect(3)</a>
<a href="zmq_send.html">zmq_send(3)</a>
<a href="zmq_recv.html">zmq_recv(3)</a>
<a href="zmq_inproc.html">zmq_inproc(7)</a>
<a href="zmq.html">zmq(7)</a></p></div>
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<div class="sect1">
<h2 id="_authors">AUTHORS</h2>
<div class="sectionbody">
<div class="paragraph"><p>This ØMQ manual page was written by Martin Sustrik &lt;<a href="mailto:sustrik@250bpm.com">sustrik@250bpm.com</a>&gt;,
Martin Lucina &lt;<a href="mailto:mato@kotelna.sk">mato@kotelna.sk</a>&gt;, and Pieter Hintjens &lt;<a href="mailto:ph@imatix.com">ph@imatix.com</a>&gt;.</p></div>
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<div id="footnotes"><hr /></div>
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<div id="footer-text">
ØMQ 3.2.5<br />
Last updated 2013-11-13 18:57:23 CET
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